This invention is generally directed to improvements in color television systems. It is particularly directed to networks for enhancing the bandwidths of relatively narrow band color signals.
Commercial television receivers typically receive a broadcast signal which includes a relatively wideband Y (luminance) signal and relatively narrow band I and Q color signals. A black and white television image is developed by applying the Y signal to a CRT (cathode ray tube). To develop a color image, the I and Q signals are demodulated to produce relatively narrow band R-Y, B-Y and G-Y color-difference signals. The latter signals are then combined with the Y signal and applied to a CRT.
Although the above-described signal processing technique generates commercially acceptable images, it is known that visible image errors are developed. For example, combining each color-difference signal with equal proportions of the Y signal at the receiver effectively attributes to each color-difference signal an equal portion of the high frequency Y components. In the general case, the primary colors do not contribute equally in the development of the high frequency Y components as the television signal is normally developed. Hence, attributing to them equal high frequency Y components at the receiver produces noticeable errors in the reproduced television image.
To overcome the problem described above, it has been proposed that, under certain conditions, each of the three color-difference signals detected in the receiver be modified so as to include its own unique, controllable portion and polarity of the high frequency components of the Y signal. U.S. Pat. No. 4,181,917 discloses that this may be accomplished by inferring that the high frequency components of the Y signal should be included in each color-difference signal according to the ratio which the derivative of each color-difference signal's low frequency components bears to the derivative of the Y signal's low frequency components. Thus, each color-difference signal is combined with a different amount and the appropriate polarity of "inferred highs" contained in the Y signal. This process is referred to herein as "chrominance bandwith enhancement".
A problem with the enhancement scheme referred to above is that it is very difficult to determine the ratio which the derivative of the low frequency color difference components bears to the derivative of the low frequency Y components. Since both derivatives approach zero near the edges of a luminance transition, it is difficult to obtain an accurate ratio determination at those times.
A related problem has to do with the effect which the bandwidth enhanced color-difference signals can have on the luminance portion of the television image. Ideally, they have no effect on it. However, because of the circuit implementation suggested in the above-mentioned patent, it is difficult to obtain bandwidth enhanced color signals which do not have an undesirable effect on the television image. Even minor variations in component tolerances can result in image distortions. Consequently, it has been difficult to include this type of chrominance bandwidth enhancement in color television receivers or in color studio equipment.